(a) Field of the Invention
The present invention relates to an objective for video disks.
(b) Description of the Prior Art
As the tracking methods for video disks, there are two main methods. One is to use a galvanomirror and to oscillate the galvanomirror, and the other is to oscillate the objective itself. In the first method, the direction of light beam, which is reflected by the galvanomirror and enters the objective, is changed by oscillating the galvanomirror. Based on the light beam of which direction is changed as above, the objective is adjusted by moving it in the direction parallel with the disk surface so that the optical axis coincides with the position of signal. In the second method, the objective itself is oscillated and the objective is adjusted in the same way as above based on the light beam which passed through the objective. When the second method is adopted out of the tracking methods described in the above, the objective should be small in size and light in weight. However, for known objective for video disks in general, N.A. is about 0.45, outer diameter is about 11 mm and weight is 3 to 5 g. In case of objectives with the above-mentioned large weight, it is not preferable to oscillate the objective itself for the purpose of tracking. However, in case of the afore-mentioned first tracking method employing the galvanomirror, it is necessary to correct offaxial aberrations of objective considerably favourably. Therefore, for known objectives for video disks to be used in the first tracking method employing the galvanomirror, the lens configuration necessarily becomes complicated and the number of lenses becomes large. Moreover, as it is necessary to make the working distance long, it is impossible to make the focal distance very small and, consequently, the outer diameter of lens necessarily becomes large. Due to the above-mentioned reasons, for known objectives for video disks, it has been very difficult to make the weight of objective smaller than the above.
On the other hand, in case of the afore-mentioned second tracking method in which the objective itself is oscillated, it is almost not necessary to correct offaxial aberrations and it is all right when paraxial aberrations are corrected. Therefore, it is possible to select the principal point, airspaces between lenses, etc. of objective comparatively freely and, consequently, it is possible to make the focal length short and other diameter small even when N.A. and working distance of objective are equal to those of known objectives.